WASHINGTON - The Obama administration fears that Syria’s embattled government may be about to use its arsenal of deadly chemical weapons against opposition forces because conventional arms have failed to halt their advance, according to the White House spokesman.

“We are concerned that an increasingly beleaguered regime, having found its escalation of violence through conventional means inadequate, might be considering the use of chemical weapons against the Syrian people,” Jay Carney, the White House press secretary, told reporters Monday afternoon.

He repeated a warning issued by Secretary of State Hillary Rodham Clinton earlier Monday, on a visit to Prague, the Czech capital, that the White House considers use of chemical weapons by President Bashar Assad’s forces a “red line” that could trigger a military response from Washington.

Later Monday, President Obama said during a speech, “I want to make it absolutely clear to Assad and those under his command: The world is watching — the use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons there will be consequences, and you will be held accountable. We simply cannot allow the 21st century to be darkened by the worst weapons of the 20th century.”

[Updated, 2:54 p.m., Dec. 3: A senior U.S. official said the United States has received new intelligence in recent days indicating that Syria’s military is making preparations for possible use of chemical weapons against rebels threatening the Assad regime.

“They are making certain preparations that have the look and feel of potential use,” the official said, speaking anonymously because he was discussing intelligence information.

He would not provide details about what steps Syria is believed to be taking or what chemical agents are involved.

“This is the Syrian military apparatus,” the official said. “This is not some splinter group.”]

Syria is known to have stockpiles of mustard gas, a blister agent, and sarin, a nerve gas. U.S. officials have said they are using satellites and other surveillance systems to ensure the poisons are not moved from bunkers.

The administration has been wary of speculating about the Assad government’s possible use of chemical weapons, so Carney’s language suggested a heightened urgency. Diplomats from other Middle Eastern countries echoed the U.S. concerns, saying that they feared rebel pressure on the outskirts of Damascus, the nation’s capital, could provoke a devastating reaction.

“As the regime has lost all legitimacy to lead Syria and the opposition grows in strength, our concern about the regime’s intentions regarding its chemical weapons stockpiles has increased,” Carney said. “We have increased concern about the possibility of the regime taking the desperate act of using its chemical weapons.”

He said Syrian government officials “must know the world is watching and that they will be held accountable by the United States and the international community.”

U.S. officials and Middle Eastern and European allies say they have been closely monitoring the chemical weapons arsenal and in recent days have been conferring on how to respond to actions by the regime.

Carney said the administration is “preparing for all scenarios” and consulting allies.

Obama also warned in August that the United States would respond if Assad used chemical weapons, or transferred them to the control of militants or other nations.

For the record, 8:45 p.m. Dec. 3: A previous version of this post said the Syrian government used poison gas against its people in the city of Hama in 1982. Some analysts say there is no conclusive proof that the government used poison gas.